The present invention relates to the manufacture of crepe paper including soft, absorbent tissue paper webs and particularly to the mode of creping of such webs to attain adequate softness and adhesive characteristics in the web while minimizing operational difficulties.
It is known in the art to form a thin paper web from a slurry of water and fiber, dewater the wet web, and then at least partially dry the dewatered web. The web is then conveyed or carried on a fabric to a large steam-heated rotary drum termed in the art a Yankee dryer. The web commonly enters the dryer at a circumferential dryer position which is a major portion around the dryer from the zone of web de-contact from the drum. The de-contact zone is equipped with a creping blade against which the web abuts so as to be pushed backwardly upon itself and attain the well-known tissue crepe paper structure.
The creping action requires that the web be well adhered to the dryer to effect a consistent and uniform creping action, and for example, to prevent flaring of the web from the dryer before or at the exit zone in the vicinity of the creping blade. In some instances the web is presented to the dryer at a considerable moisture content that is typically as high as about 60%. Such webs accordingly have fiber consistencies at the point of contact with the dryer of about 40%. The moisture content, depending upon the condition of the web surface and the Yankee dryer surface, may tend to cause the web to adhere strongly to the dryer throughout the drying action of the rotating drum. Under such circumstances, there is usually no requirement for the use of a supplemental adhesive, and on some occasions the adhesion to the dryer is so tight that a release agent is applied between the dryer and the web to limit the extent of adhesion.
In some modes of operation commonly referred to as through-drying, contact of the web with the dryer surface is limited. In a through-drying operation, the web formed from the slurry of water and fiber is dewatered without significantly pressing the wet web. This is followed by a drying action in a hot air blast. The resulting webs are then pressed to the Yankee dryer using a knuckled fabric so that the web adheres to the dryer in closely spaced zones, with bulking of the web between the zones. Fabrics having as fine a count as 4,900 openings per square inch and above may serve the purpose. The fiber consistency of such webs when presented to the dryer may be from about 30% to about 90% fiber. Higher fiber-consistency webs commonly require an adhesive to adequately secure the web to the dryer for completion of both the drying action and creping action.
A variety of adhesives have been employed for retaining a web on a dryer surface. These include polyvinyl acetate-ethylene copolymer emulsions and aqueous polyvinyl alcohol solutions. It has been found that polyvinyl acetate-ethylene copolymer compositions that may contain small percentages of polyvinyl alcohol such as less than about 5% of the total solids by weight, are generally adequate for the purpose but cause a number of undesirable effects. Polyvinyl alcohol adhesives pose similar problems.
With more specific reference to the polyvinyl acetate-ethylene copolymers, these copolymers tend to be retained on the creped paper web on the side that engages the dryer. When a resulting creped web is rolled or rolled and plied with another web to form a sheet, the surface or surfaces carrying the residual adhesive tends to adhere to the adjacent rolled opposite surface causing blocking in the roll. Such blocking has additional detrimental effects in light-weight tissues where if any minor edge defect or small hole occurs, an effort to separate the blocked plies can result in tear-outs from the web, complete web breakage, and even ply reversal, that is, the attachment of a ply to an adjacent sheet.
Additionally, it has been found that the fabric carrying the web to the dryer may become contaminated with the copolymer that is difficult to remove with conventional cleaning actions, particularly due to the water-resistance of the copolymers.
In contrast to the polyvinyl acetate-ethylene copolymer, polyvinyl alcohols having a significant degree of hydrolysis tend to be water-soluble and also tend to be unaffected by most organic solvents. The polyvinyl alcohols (which may contain some polyvinyl acetate) tend to coat the dryer with a hard and uneven film that builds up as drying and creping proceed, resulting in uneven creping. Additionally, to remove the hard film, a cleaning blade is frequently used against the dryer surface causing dryer surface wear.
These and other problems of the aforementioned systems, methods, and adhesives, are overcome by the creping adhesives, systems, and methods of the present invention.